1. Field of the Invention
The present invention relates to an image processing method and image processing apparatus, which divide an image obtained by scanning a paper manuscript into regions, and convert the image into vector data for respective divided regions.
2. Description of the Related Art
In recent years, along with the advance of digitization, a system which stores digital data obtained by converting paper documents and transmits the digital data to other apparatuses instead of holding the paper documents intact is prevailing. Documents to be converted into digital data include not only monochrome binarized images, but also digital documents such as full-color (multi-level) documents. Furthermore, digital documents to be converted are not limited to image data obtained by scanning paper documents using a scanner or the like. For example, digital documents include data converted into more advanced information (e.g., a document image is divided into regions, a region of text undergoes character recognition processing to be converted into a character code sequence, a region of a photographic image is converted into outline vector data, and so forth) (see Japanese Patent Laid-Open No. 2004-265384 (WO2004/068368)).
The aforementioned images to be vector-converted include those which are obtained by scanning an illustration or a manuscript created by graphics creation software although they are full-color images. These images are characterized in that they are clear since the outlines of objects are rimmed compared to natural images such as photographic images, and the number of colors of appearance is limited, and so forth. In the following description, such an image will be referred to as a “clipart image”.
Conventionally, image vector conversion processing is executed as follows. An input full-color image is converted into a binarized image, outline data and central line data are extracted from the binarized image, and the obtained outline data and color information of an original image are converted into vector data. Also, a method of dividing an image into regions by using color features, and converting outline data and inner colors of the region dividing result into vector data is available.
Furthermore, conventionally, various kinds of image processing for images including a gradation have been examined. For example, an image processing apparatus, which is described in Japanese Patent Laid-Open No. 10-243250 (U.S. Pat. No. 6,128,407), detects an area of gradation from a color image, reproduces the detected area of gradation as gradation, and applies, to a flat color area, image processing suited to flat color, so that the color image falls within a color reproduction range. The apparatus calculates variance values of an LCH color space of a region so that the area of gradation is expressed by the same color, detects a larger region on the color space as gradation, and reproduces this area of gradation intact.
An image change detection apparatus described in Japanese Patent Laid-Open No. 2001-309373 (US Patent Pub. No. 2001/0033691) executes feature extraction by second-order differentiation of brightness signal values corresponding to a plurality of pixels in a horizontal line of those which form a still image, so as to detect an area of gradation in the still image. When the result obtained by second-order differentiation is zero, the apparatus detects that the area of gradation is included.
However, when a clipart image including gradation undergoes the aforementioned vector conversion based on outline extraction, a problem that the area of gradation may be divided into many regions, a problem that the area of gradation may be divided into inappropriate clusters, and the likewise. Also, even with the method based on region division, if an area of gradation is divided into one region, a problem that achromatic and chromatic color areas may be included in an identical cluster arises. These problems cause an increase in vector data size and deterioration of image quality of a vector image.
Furthermore, the conventional color reproduction processing and image change detection processing for an image including gradation merely pertain to the detection of an area of gradation.